1. Field of the Invention
The present invention relates generally to wireless charging, and more particularly, to a method for detecting a load in a wireless power transmitter.
2. Description of the Related Art
In view of their nature, mobile terminals such as portable phones and Personal Digital Assistants (PDAs) are powered by rechargeable batteries. To charge the batteries, the mobile terminals apply electric energy to the batteries through chargers. Typically, the charger and the battery each have an exterior contact terminal (i.e., the contact terminals protrude outward) and thus are electrically connected to each other by contact between their contact terminals.
This contact-based charging scheme faces the problem of the contact terminals becoming contaminated with foreign materials, which can result in unreliable battery charging. Moreover, if the contact terminals are exposed to moisture, the batteries may not charge properly.
To address the above problem, wireless charging or contactless charging technologies have recently been developed and are applied to many electronic devices.
Such a wireless charging technology is based on wireless power transmission and reception. For example, once a portable phone is placed on a charging pad, without being connected to an additional charging connector, its battery is automatically charged. Among wirelessly charged products, wireless electric toothbrushes or wireless electric shavers are well known. This wireless charging technology offers the benefits of increased water-proofness due to the wireless charging of the electronic products and enhanced portability due to no need for a wired charger for electronic devices. Further, it is expected that various relevant wireless charging technologies will be more developed in the upcoming era of electric vehicles.
There are three wireless charging schemes largely: a) electromagnetic induction using coils, b) resonance-based, and c) Radio Frequency (RF)/microwave radiation based on conversion of electric energy to microwaves.
So far, the electromagnetic induction-based wireless charging scheme has been dominantly popular. However, considering recent successful experiments in wireless power transmission over microwaves at a distance of tens of meters, it is foreseeable that every electronic product will be charged cordlessly at any time in any place in the near future.
Electromagnetic induction-based power transmission means power transfer between primary and secondary coils. When a magnet moves through a coil, current is induced. Based on this principle, a transmitter creates a magnetic field and a receiver produces energy caused by current being induced by a change in the magnetic field. This phenomenon is called magnetic induction and power transmission based on magnetic induction is highly efficient in energy transfer.
Regarding resonance-based wireless charging, a system that makes wireless energy transfer from a charger at a distance of a few meters based on the resonance-based power transmission principle by the Coupled Mode Theory has been proposed. The MIT team resonated electromagnetic waves carry electric energy, instead of sound. The resonant electric energy is directly transferred only in the presence of a device having the same resonant frequency, while the unused electric energy is reabsorbed into the electromagnetic field rather than it is dispersed in the air. Thus the resonant electric energy does not affect nearby machines or human bodies, compared to other electronic waves.
Wireless charging is actively being researched. Thus, there is a need for developing a standard regarding wireless charging priority, detection of a wireless power transmitter/receiver, communication frequency selection between a wireless power transmitter and a wireless power receiver, wireless power control, selection of a matching circuit, and allocation of a communication time to each wireless power receiver in a single charging cycle.
Further, there is a need for a method for effectively detecting a load in a wireless power transmitter (that is, a Power Transmitting Unit (PTU)).